Add timezone selector to Admin settings with full-stack support

- Add `timezone` column to User model (VARCHAR 64, default UTC) with
  idempotent startup migration
- Expose and persist timezone via PATCH /api/users/me
- Fix TimerEvent.occurred_at serialization to include UTC offset marker
  (+00:00) so JavaScript correctly parses timestamps as UTC
- Add frontend utility (src/utils/time.js) with timezone-aware
  formatTime, getHHMM, getDateInTZ, tzDateTimeToUTC helpers and a
  curated IANA timezone list
- Add Settings section to Admin page with timezone dropdown; saves to
  both the API and localStorage for the unauthenticated TV view
- Update Activity Log to display and edit times in the user's timezone
- Update TV dashboard clock to respect the saved timezone
- Update README: features, setup steps, usage table, WebSocket events

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/TVView.vue

@@ -112,15 +112,18 @@ const route = useRoute()
 const scheduleStore = useScheduleStore()
 const childId = parseInt(route.params.childId)
 
+// Read timezone from localStorage (set by the parent in Admin → Settings)
+const tz = localStorage.getItem('tz') || undefined
+
 // Clock
 const now = ref(new Date())
 setInterval(() => { now.value = new Date() }, 1000)
 
 const clockDisplay = computed(() =>
-  now.value.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit', second: '2-digit' })
+  now.value.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit', second: '2-digit', timeZone: tz })
 )
 const dateDisplay = computed(() =>
-  now.value.toLocaleDateString([], { weekday: 'long', month: 'long', day: 'numeric' })
+  now.value.toLocaleDateString([], { weekday: 'long', month: 'long', day: 'numeric', timeZone: tz })
 )
 
 // Day progress helpers